Lignocellulosic material in fiber form is in wide commercial use as a raw material used for the manufacture of paper and other paper products. In paper making, wood fibers that are obtained from pulping fiber source materials usually are treated by combining them with other additives, and the fibers are then processed into a network of wood fibers, which can constitute a paper or other thin sheet of fibrous material.
Wood contains pitch and/or other extractives in addition to its main components cellulose, hemicelluloses, and lignin. Pitch refers to a variety of naturally occurring, hydrophobic, organic resins of low and medium molecular weight in wood fiber which include esters of fatty acids with glycerol (such as the triglycerides), as well as other fats, fatty acids, sterols, steryl esters, resin acids, and waxes, and to the deposits that these resins can cause during pulping and paper making processes. Pitch is typically located in parenchyma cells and on the surfaces of the fiber. Pitch composition can vary depending on the season and the type of wood, and some wood species may create greater pitch problems during pulping and papermaking than other species. Generally, pitch content from about 1 wt % to about 10 wt % can be encountered for virgin wood fiber used in paper making processes. Based on solubility in ethyl ether values, for example, pitch may comprise, for example, from about 0.7 to about 2.4 weight percent of hardwoods such as beech and white birch, and from about 0.7 to about 4.3 weight percent of softwoods such as eastern hemlock and jack pine, based on the total weight of unextracted (oven-dry) wood.
Pitch and other deposit-forming organic contaminants from wood fibers, even if only present in relatively small amounts, can cause problems in paper making systems that are significant and disproportionately larger than their relative concentrations. Pitch deposits may occur throughout a pulp or paper mill and these deposits can both degrade product quality and impair production rates. Pitch can cause problems in paper machines, for example, by sticking to rollers or other process equipment, and causing spots or holes in the paper material. Pitch also can impair production rates by decreasing the efficiency of pulp washing, screening, centrifugal cleaning, and refining, and disrupting many paper machine operations. Pitch can degrade the product paper by causing spots, holes, picking, and scabs in the final paper product or sheet. Pitch deposition on process equipment in paper making systems using virgin pulp supplies also can result in operational problems and production inefficiencies. Pitch can deposit, for instance, on screens used in the process line to reduce their throughput, and/or on process control devices, rendering them inoperable, such as instrument probes. Deposition of the pitch can occur not only on metal surfaces in the system, but also on plastic and synthetic surfaces such as machining wires, felts, foils, uhle boxes and headbox components. Pitch deposits may also break off resulting in spots and defects in the final paper product which decrease the quality of the paper. Pitch deposits have been removed from process equipment in paper mills, for example, by periodic boil-outs, solvents, or mechanical dislodgement, to maintain cleanliness of equipment and mill systems. Boil-outs can be done, for example, using caustic soda, surfactants, and water at elevated temperatures. The need for such cleanings to remove pitch deposits on equipment typically requires shutdown, and reduces productivity and increase production costs. In addition, production trends may increase pitch problems, for example, the use of high-speed machines that create high shear rates, or higher production rates that increase the load on washing equipment and thus increase the concentration of pitch in the stock, or the reuse of white water and more complete closure which may concentrate pitch and aggravate pitch deposition, such as in bleach plants, and the like.
Past efforts to control pitch problems have widely varied. Pitch control measures have included aging or seasoning wood, the use of wood species with low resin contents, and the modification of pulping parameters. Modifications in pulping parameters may include process variables such as pH, temperature, first-pass retention, washing efficiency, bleaching agent, and the use of process additives, such as cationic polymers, alum, and talc, which have been employed to control pitch problems. For example, the addition of lipase and a cationic polymer to a cellulosic slurry for pitch deposit control has been mentioned. See, e.g., U.S. Pat. No. 5,256,252.
The present investigators have recognized that improved pitch control strategies are useful for paper making processes to reduce pitch deposition on process equipment and paper products to reduce mill equipment cleaning and maintenance requirements, increase product quality, or both. The present inventors further have recognized a need to control pitch and other resin deposit-forming constituents of wood fiber by a treatment of the fibers with a combination of agents which can reduce the content of pitch or other deposit-forming constituents of fiber in a way not predicted from the effects of the individual components.